1. Field of the Invention
The present invention relates to a flow rate detection device for measuring the volume of air.
2. Description of the Related Art
Until now, as a device for measuring the volume of air in an internal combustion engine, for example, there has been known a flow rate detection device employing an inflow temperature-dependent resistor. The flow rate detection device is provided with a sensor element including a heating element and an intake air temperature detector. The sensor element is kept under control so that a temperature of the heating element is higher than a temperature detected by the intake air temperature detector by a given amount, so as to detect a voltage corresponding to an amount of heat that the heating element dissipates into a fluid, to thereby detect a flow rate of the fluid finally.
Japanese Patent Application Laid-open No. 09-026343 discloses an example of a support structure for the sensor element used in such a flow rate detection device. The sensor element disclosed in Japanese Patent Application Laid-open No. 09-026343 includes a cavity, which is formed in a rear surface of a plate-shaped semiconductor silicon substrate by removing part of the substrate, and a thin film portion, which is disposed over the cavity and includes a detecting element. The sensor element is bonded to a support fitting portion only on one side, that is, the sensor element is supported in a cantilever manner.
In the flow rate detection device having the cantilever support structure as disclosed in Japanese Patent Application Laid-open No. 09-026343, a fluid to be measured flows only on a front surface of the sensor element in a low flow rate range, whereas in a high flow rate range, the fluid to be measured flows into a gap between the sensor element and the support fitting portion as well (hereinafter, this flow is referred to as “undercurrent”), which leads to a problem of reduced accuracy in flow rate detection. In order to solve the problem, in Japanese Patent Application Laid-open No. 09-026343, a groove-shaped slot is provided in the support fitting portion along the periphery of the sensor element so that the undercurrent, which occurs in the high flow rate range, may be prevented from directly contacting the sensor element.
However, when the fluid to be measured is guided into the slot, part of the fluid sometimes flows around and into the gap between the sensor element and the support fitting portion, and the undercurrent cannot be prevented sufficiently.